OC-11702 - handle missing groups during authzid mapping
OC-11702 - handle missing groups during authzid mapping

This commit fixes a bug where opscode-account throws an
exception when trying to map the authz_id for a group that
no longer exists to a name. The bug manifests itself in two
separate ways:

* when a showing a group that contains a group that
  has been deleted
* when showing the ACLs for an object that contains a
  group that has been deleted

chef/mixlib-authorization